This invention relates to a silver halide photographic emulsion and, more particularly, to a silver halide photographic emulsion supersensitized with a combination of at least two sensitizing dyes to impart a high red sensitivity.
Silver halide photosensitive materials are each required to be highly sensitive to a particular wavelength region of the spectrum depending upon each purpose of use. It is well known that as one of the techniques to produce such photosensitive materials, a certain type of sensitizing dye is added to the silver halide emulsion to enhance effectively the sensitivity to a specific region of the spectrum where the wavelengths are longer than those of the region in which the silver halide exhibits intrinsic sensitivity. It is also known that when used in combination with another sensitizing dye or a special organic compound, such a sensitizing dye imparts to the emulsion a sensitivity larger than the sum of sensitivities imparted by each sensitizing dye or organic compound alone. Such an effect is called "supersensitization", and many combinations have already been reported. Since the silver halide photographic sensitive materials in recent years are required to be more sensitive, it is important to develop a technique for performing the spectral sensitization more effectively. In order to produce photosensitive materials of high sensitivity, it is advantageous to use a combination of at least two types of sensitizing dyes which are in supersensitizing interrelationships and which do not accompany densensitizing effect.
Furthermore, with the rapid progress in optoelectronics, laser beams and light rays from LED are used as light source in place of conventional incandescent lamps in image processing by the electrooptical conversion of signal current. Particularly, wavelengths of the rays emitted from He-Ne laser, ruby laser, and red LED are in the spectral region of from 600 to 700 nm. For the sensitive materials used to record such red rays, conventional supersensitizing combinations are insufficient in sensitivity.